The reaction: EQU Alk--OH+HX.fwdarw.Alk--X+H.sub.2 O
in which X represents a halogen atom, is generally known to take place. It is, however, considered to conform in type to the process of esterification, in which equilibrium between reactants and products is attained, as described by Migrdichian ["Organic Synthesis", Vol. 1, p.17, 1957, Reinhold Publishing Corp]. In practice, the preparation of alkyl halides in aqueous solution of the acid has been impractically slow, inefficient and in some cases even impossible to achieve. For this reason, even though aqueous HX is a very convenient form of HX and often the cheapest form available, the art has so far not provided an efficient process for the production of alkyl-halides in aqueous solutions of the acid, in good yield and with industrially acceptable rates. Thus Jolles ["Bromine and its Compounds", Ernest Benn Ltd., 1966, p.377] reports the preparation of 2-bromopentane in only 89% yield from the corresponding alcohol and 54% aqueous HBr in a 4 hour reaction in which two-thirds of the HBr must subsequently be discharged as waste. Norris [Am. Chem. J., 38, 627-42 (1907)] obtained similarly low yields of alkyl bromides by the slow distillation of product from mixtures of the corresponding alcohols in excess aqueous HBr, but under such conditions primary alcohols did not react with aqueous HCl.
In the case of such slow reactions with the lower alcohols, one cannot conceive to distil out the product alkyl halide and water as formed, and thus hope to maintain a constant acid concentration, because the alcohols themselves are very volatile and most of them form low boiling azeotropes with water and would thus be distilled out of the reactor before reacting. This is a serious limitation when operating with the process described by Norris. With methanol, the yield of CH.sub.3 Br obtained by Norris was only 50%.
To promote such reactions it is common practice either to use anhydrous HX and operate in the vapor phase, or/and to employ dehydrating agents. Thus, sulfuric acid is commonly used for this purpose, sometimes both as a means of generating HX from a convenient salt (thus operating in an almost anhydrous mode) as well as to serve as a condensation agent. ZnCl.sub.2 is commonly used in the case of CH.sub.3 Cl manufacture in the liquid phase, and alumina often serves as catalyst for the manufacture of this material in the vapor phase [see, e.g., Weissermel and Arpe, "Industrial Organic Chemistry", Weinheim (1978), pp. 46-50].
It should be borne in mind that aqueous HX, on reacting in such processes, not only produces one mole of water for each mole of acid reacted, but at the same time liberates all of the water molecules which were associated with it in the original acid solution. Thus, on using 48% aqueous HBr, for each mole reacting 6 moles of water are liberated, which dilute the unreacted acid. The serious influence of the phenomena on reducing the reactivity of the remaining acid is immediately obvious.
In the cases of methyl bromide and methyl chloride, these can be produced by the halogenation of methane. This process is of industrial use in the case of methyl chloride if one has available a source of pure methane gas and can use the large amount of by-products (higher chlorinated methanes) coproduced [Weissermel and Arpe]. The corresponding reaction with bromine is not industrially useful.
Alkyl bromide can be produced by reacting the alcohol with elemental bromine and either phosphorus or sulfur. Elemental bromine is more expensive than HBr which is obtainable as a by-product in bromination reactions. Furthermore, the cost of using the phosphorus or the sulfur is increased even more, since these are converted into the corresponding acids in the reaction and become ecologically troublesome wastes. This reaction is nevertheless employed in the industrial manufacture of methyl bromide.
In spite of the failure of previous attempts to obtain high yields and rapid conversions of alkanols to alkyl halides in aqueous HX, it has now been found that it is possible to obtain the desired result if both the acid concentration and the temperature are high enough, and the instantaneous molar ratio of HX to alkanol is greater than 3. This is accomplished by continuously feeding HX and a lower alcohol to a reactor containing aqueous HX maintained under the appropriate conditions and lower alkyl-bromide and water are continuously distilled off from the said reactor, the lower alkyl-bromide and water being continuously separated, and part of the said water being recycled to the distillation column to abate HBr distillation from the reactor.